January 10, 2019

What is the role of the infrared filter?

The correct name for the filter on the surveillance camera CCD is called “optical low-pass filter”

The filter has two functions: 1. Filter out the infrared light. 2. Trim the incoming light.


1. Filter out the infrared:

The surveillance camera color CCD can also sense infrared light, because it will induce infrared rays, which will cause the DSP to not calculate the correct color. Therefore, a filter must be added to separate the infrared part of the light, so only the color CCD needs to be equipped with a filter. Black and white will not be used.

2. Finishing the light:

Because the CCD is composed of a single photoreceptor (CELL), it is best to direct the light, but in order to avoid interference with the adjacent photoreceptor, the light needs to be trimmed, so the filter is not glass, but quartz. The film, using the physical polarization characteristics of quartz, keeps the incoming light, retains the direct part, and reflects off the oblique part to avoid affecting the photosensitive spot next to it.

So how do the filters do this?

1 Filter out the infrared:

The coating method and the blue glass can be used, and the coating is divided into a vacuum coating method and an electroless plating method. The electroless plating method is to immerse the quartz sheet in a solvent for electroplating, the cost is low, but the coating thickness is uneven and easy to fall off, and the vacuum coating is vacuum evaporation, and the coating is uniform. It is not easy to fall off, but the cost is high. Above we call IR Coating, the purpose is to filter out the infrared rays, and also add the so-called AR-Coating coating, the purpose is to increase the light transmittance, because the light is transmitted through different media (such as From the air into the quartz plate), part of the refraction and reflection will be produced. After AR-Coating, the filter can achieve a penetration rate of 98-99%, otherwise it will only have a transmittance of 90-95. Sensitivity certainly has an effect.

In addition, it uses blue glass. Blue glass filters infrared rays by means of “absorption”. IR-Coating filters out infrared rays by reflection, but reflected light is easy to cause interference. If only infrared rays are filtered out, blue glass is better. The choice. But the above said that the glass can not trim the light, so there is a so-called “two-piece” filter with a piece of blue glass and a piece of quartz. The blue glass is used to filter the infrared rays, and the quartz piece is used to trim the light, so the quartz Just do AR-Coating on the chip.

2. Trimming the light:

As mentioned above, using the physical polarization characteristics of quartz, the incoming light remains in the direct part and reflects off the oblique part, but can only be trimmed in one direction. Usually the camera only considers the horizontal resolution, so only the light is made. Horizontal trimming, so the direction should be correct when the filter is attached, can not be reversed. If the vertical light should also be trimmed, what should I do? Very simple, stick two pieces, turn one of them into 90 degrees, so there is This kind of filter, also called “two-piece”, is used for horizontal trimming, one for vertical trimming, and one for IR-Coating to filter infrared.

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